The vast majority of current refrigeration systems are based on vapor compression technology, thus applying compression and expansion of a fluid refrigerant. Since many refrigerants have large ozone depletion potential, and are often seen as a possible factor in global-warming, a significant effort has been dedicated lately to the development of alternative refrigeration technologies.
Various electrocaloric materials (ECMs) are known. Such ECMs react to changes in the strength of an electric field, to which they are exposed, by changing their temperature. ECMs increase in temperature upon increase of the electric field and they decrease in temperature upon reduction (or removal) of the applied field. With the recent discovery and development of improved electrocaloric materials (the improvement lying in an increased temperature difference effected by the change in electric field strength) this technology has attracted further attention in the refrigeration field. The art has recognized that the electrocaloric effect can be applied not only for refrigeration, but also for the other types of processes, such as air conditioning, heat pump technology, and power generation.
The use of ECMs in refrigeration systems is known. Known concepts use cascading thermal bridges including electrocaloric materials as the refrigerant and thermal diodes (or thermal switches) as the heat transfer mechanism. For example U.S. Pat. No. 4,757,688 and US 2011/0113791 A1 use a “heat pipe” as a thermal switch between heat source and heat sink and/or between different stages of the ECM. US 2010/0175392 A1, on the other hand, uses liquid crystal thermal switches. US 2011/0146308 A1 describes the use of ECM in a form of a membrane which is alternatively disposed between heat source and heat sink, thereby selectively transferring the heat. In U.S. Pat. No. 6,877,325 B1 a single stage and a cascade electrocaloric device using heat exchangers between successive stages is disclosed. U.S. Pat. No. 6,877,325 B1 also provides a concept in which a working fluid passes through the electrocaloric material. However, this system cannot provide internal heat regeneration (as defined below), since it works as a single stage or as a cascade device, and does not apply a working fluid in counter flow operation.
US 2012/0055174 A1 discloses systems for transferring heat from a heat source to a heat destination including one or more electrocaloric heat pumps based on an electrocaloric material thermally and mechanically coupled to both ends of a heat pipe.
WO 2006/056809 A1 discloses a cooling device comprising several electrocaloric working elements separated by heat switches in a cascaded or paralleled arrangement to improve the cooling effect.
WO 2011/0113791 A1 discloses systems and processes for thermal management using the electrocaloric effect. An electrocaloric element is interposed between an electronic component (heat source) and a heat pipe, wherein the heat pipe is also thermally coupled to a heat sink.
All electrocaloric refrigeration systems known to date suffer from relatively large heat transfer losses and they can only produce a relatively limited temperature difference between heat source and sink. Such systems are not deemed competitive with vapor-compression technology. The large heat transfer losses occur, because in the known systems (such as the ones described in U.S. Pat. No. 4,757,688 and US 2011/0113791 A1) a cascade connection of ECM is applied in order to increase the temperature difference. Since the adiabatic temperature difference of a single stage of ECM is rather small, a useful device (producing a sufficiently high temperature difference of, e.g., 35 K) will require ten or more cascades. This leads to a large loss of energy. On the other hand a small number of cascades will not produce a sufficiently large temperature difference.
Against this background, the present inventors have developed a system and methods for electrocaloric energy conversion, which systems and methods overcome the above shortcomings. The invention is based on the use of ECM and selective heat transfer mechanisms, which enable heat to be selectively transported from an ECM towards a first stream of working fluid (flowing towards a heat sink) and from a second stream of working fluid towards the ECM (said second stream flowing towards a heat source). An important aspect of the first and second streams of working fluid is that they are in counter flow. Preferably, said first and second streams of working fluid are thermally coupled and in counter flow. By using ECM as an internal regenerator between said first and second streams of working fluid, said streams being in counter flow, rather large temperature differences can be established between heat source and sink. The present invention is based on a concept of combining internal heat regeneration (through working fluid flowing in counter flow) with the generation of temperature differences using electrocaloric materials. Such internal regeneration of heat energy (IRE) has not been applied in electrocaloric energy conversion systems so far.